The present invention generally relates to an illumination system comprising a radiation source and a luminescent material comprising a phosphor. The invention also relates to a phosphor for use in such an illumination system.
More particularly, the invention relates to an illumination system and luminescent material comprising a phosphor for the generation of specific, colored light, including white light, by luminescent down conversion and additive color mixing based on an ultraviolet or blue radiation emitting radiation source. A light-emitting diode as a radiation source is especially contemplated.
Recently, various attempts have been made to make white light emitting illumination systems by using visibly colored light emitting diodes as radiation sources. When generating white light with an arrangement of visibly colored red, green and blue light emitting diodes, there has been such a problem that white light of the desired tone cannot be generated due to variations in the tone, luminance and other factors of visibly colored light emitting diodes.
In order to solve these problems, there have been previously developed various white light illumination systems, which convert the radiation of UV to blue light emitting diodes by means of a luminescent material comprising a phosphor to provide a visible white light illumination.
Phosphor converted white light illumination systems have been based in particular either on the trichromatic (RGB) approach, i.e. on mixing three colors, namely red, green and blue, in which case the latter component of the output light may be provided by a phosphor or by the primary emission of the light emitting diode or in a second, simplified solution, on the dichromatic (BY) approach, by mixing yellow and blue colors, in which case the yellow secondary component of the output light may be provided by a yellow phosphor and the blue component may be provided by a phosphor or by the primary emission of a blue light emitting diode.
In particular, the dichromatic approach as disclosed in, for example, U.S. Pat. No. 5,998,925 uses a blue light emitting diode of InGaN based semiconductor material combined with an Y3Al5O12:Ce (YAG-Ce3+) phosphor. The YAG-Ce3+ phosphor is provided in a coating on the InGaN LED, and a portion of the blue light emitted from the LED is converted to yellow light by the phosphor. Another portion of the blue light from the LED is transmitted through the phosphor. Thus, this system emits both blue light, emitted from the LED, and yellow light emitted from the phosphor. The mixture of blue and yellow emission bands is perceived by an observer as white light with a typical CRI in the middle 70 ties and a color temperature Tc, that ranges from about 6000 K to about 8000 K.
Recent advances in manufacturing nitride-based LEDs have resulted in highly efficient light sources, and their efficiency is expected to surpass filament and fluorescent based light sources. Yet, all over efficiency is a recognized problem with phosphor-converted illumination systems, especially systems comprising light emitting diodes as their radiation source.